Process Capability
Index

A common Process Capability measure, Cp (often
called a Process Capability Index), indicates how well the process
distribution fits within its specification limits, and is simply the ratio
of the specification width to the variation width.

The problem with Cp is that it does not take
account of how well the process distribution is centered within its
limits, which can result in a process with both a low Cp and many
rejects. The solution to this is a second measure, Cpk, which
measures a similar ratio, but considers only the variation half that is
closest to the specification limits. Thus Cp and Cpk, taken
together, give a measure of both the potential and centering of the
process distribution within the specification limits.